Packers are used downhole for isolating zones in a wellbore. There are several styles available. Some of the more established designs feature a hydraulically or mechanically set design involving one or more sets of slips that are ramped out for anchoring the packer body while the sealing element is compressed to extend radially to the surrounding tubular with the slips holding the set position. Another style that has been used is the inflatable which has been run into casing, through tubing and in open hole applications. In this design the element has a fixed end and a movable end so that the fluid that inflates the element causes it to get larger radially for sealing contact while shrinking longitudinally. A system of valves retains the charging fluid and prevents overpressure of the element. Other designs have a sealing element on the outside of the mandrel and use radial expansion from within the mandrel. Some designs more recently have started using materials that swell in response to exposure to fluids already in the wellbore or added later on. Other designs have used shape memory materials to take advantage of the ability of such materials to revert to different shapes when raised above the transition temperature. Shape memory alloys are selected for the sealing element or other components such as springs that act on the slips or sealing element when the packer is set.
Among the swelling packers there continues to be a problem with delay of swelling long enough to allow the swelling packer to be properly positioned before it has swollen so much that it cannot be advanced to the proper location because it has already swollen too much. One solution that has been tried is to put an outer layer of a material that swells less than the core material and regulate the rate that well fluids diffuse through the outer layer before they can reach the core that swells at a significantly faster rate. This design is illustrated in U.S. Pat. No. 7,143,832. Some packers feed the fluid for swelling through the tubing instead of the well annulus where the packer is set. These designs also have a cover on the exterior that serves as the sealing component. There is no active external force that compresses the swelling element that ultimately drives the surrounding element into sealing contact in the wellbore. These designs are shown in U.S. Pat. Nos. 2,849,070 and 2,945,941. More recently in November 2007, the inventors of the present invention invented a coating for a swelling element packer with a resin with hardener fiberglass material known as Bondo® Product No. 402 made by the Bondo Corporation a subsidiary of 3M Corporation that is available at Wal-Mart®, a nationwide and international chain of retail general stores. This product is a fiberglass resin that is mixed with a hardener and was brushed on without the fiberglass mat material as a test of whether it would delay swelling. It was allowed to air dry before running the packer into the well. This technique was labor intensive and the fiberglass epoxy resin had a very short pot life making the consistency of the coating variable and while having some benefit in retarding swelling the nature of the application process made it difficult to determine if there were beneficial results even after multiple runs with such a coating. Other packers have used a cover on a compression set sealing element. In this application the packer sealing element was selected for resistance to hydrogen sulfide and the material chosen also had the characteristic of swelling in oil based mud. The cover was placed over the sealing element exterior and when the destination for the packer was reached the sealing element was longitudinally compressed with the intent that the cover would be mechanically compromised as the sealing element was forced out. The fact that the element swelled was a downside in a decision controlled by tolerance to corrosive materials in the well and played no part in the actual sealing process. This type of packer is illustrated in U.S. Pat. No. 4,862,967.
The present invention seeks to make a swelling packer operate in high temperature applications by covering it externally with an impervious cover to allow sufficient time to get the packer to the desired position before it swells to a point where it can not be advanced. The coating is preferably formed into a tubular shape and shrink fit to the swelling element. On the way down to the desired location the combination of the well temperature and the well fluids attack the cover to the point that ideally when the packer is placed at the desired location in the well the cover has been attacked sufficiently for it to have to have dissolved or have pieces break off or will break down in a matter of a few hours or days so that the sealing due to swelling results in contact of the core material with the surrounding tubular or the wellbore wall in open hole. The material for the cover is expected to lose its integrity as opposed to remain intact and simply control well fluid diffusion through it. The cover, when applied is impervious and its surrounding fluid environment causes it to fail by several potential mechanisms. These and other aspects of the present invention will be more apparent to those skilled in the art from the description of the preferred embodiment described below and the associated drawings, while recognizing that the full scope of the invention is determined by the claims.